The present invention relates to a plasma display apparatus (PDP apparatus) that performs a gradated display using a subfield method and, more particularly, to a technique for improving the display quality of a PDP apparatus.
The plasma display apparatus (PDP apparatus) has been put to practical use as a flat display and is a promising thin display of high-luminance. In the PDP apparatus, as it is possible only to control each display cell to be lit or not, a display frame is made to consist of plural subfields and the subfields to be lit are combined in each cell to perform a gradated display. Each subfield comprises at least an address period during which a display cell is selected and a sustain period during which the selected cell is lit. In the sustain period, a sustain pulse is applied to cause a sustain discharge to occur, and the luminance is determined by the number of sustain pulses. In the following explanation, the total number of sustain pulses in each subfield, that is, the number of sustain pulses that can be applied to each cell in one display frame, is referred to as the total sustain pulse number. If the cycle of the sustain pulse is the same, the luminance is determined by the length of the sustain period. Although the most general and efficient configuration of the subfield is that in which the lengths of the sustain periods in the subfields serially increase and the ratio of the length, that is the luminance, of the sustain period in a subfield to that of the previous one is 2, various subfield configurations have been proposed recently in order to suppress false contours. The present invention can be applied to a PDP apparatus that performs a display using any subfield configuration.
Moreover, various methods have been proposed for the PDP apparatus, and the present invention can be applied to a PDP apparatus that employs any method. As the configurations and the driving methods of the PDP apparatus are widely known, a detailed description is not given here.
One of the problems of a PDP apparatus lies in that the ability to perform the gradated expression is insufficient and particularly, the ability to express low gradations is insufficient. This is because the number of subfields that can be processed in one display frame period is limited.
Techniques for performing the gradated expression without increasing the number of subfields include a method for generating a pseudo-intermediate gradation by the error diffusion process. However, if the error diffusion process is performed, a problem is caused in that dot-like noises become conspicuous particularly in a low-gradated display. This is because the difference in luminance between neighboring gradations is large and the noises are particularly conspicuous in low gradations in which the difference in luminance between neighboring gradations seems to appear relatively large. If the difference in luminance between neighboring gradations is reduced while maintaining the same number of subfields, the peak luminance is lowered, therefore, it is necessary to increase the number of subfields in order to reduce the difference in luminance between neighboring gradations while maintaining the same peak luminance.
Techniques for increasing the number of subfields include a method for increasing the number of subfields, in which a screen is divided vertically into two and driven, thereby the address period is shortened and the shortened periods are combined. However, in order to employ this method, it is necessary to provide an address driver and a sustain drive circuit respectively in the upper and lower screens, therefore, a problem is caused in that the cost and the power consumption are increased.
U.S. Pat. No. 6,414,657 has disclosed the technique for adjusting at least one of the number of gradations, the constant doubling factor, the number of subfields, and the weighting multiple by calculating the amount of false contour noises from the detected movement. To be specific, the configuration in which the number of subfields is increased/decreased according to the average level/peak level of the entire screen has been described, and in this configuration, the number of subfields is increased when the average level of the entire screen is high.
Moreover, U.S. Pat. No. 6,686,698 has described a configuration, in which, after attention is paid to the fact that the display quality is not degraded even when the cycle of a sustain pulse is shortened if the subfield has a low display load ratio, the display load ratio is detected for each subfield, the cycle of a sustain pulse is shortened only in a subfield having a low display load ratio, the total of vacant times generated by shortening in the display frame is redistributed to each subfield, and thus the total number of sustain pulses is increased to increase luminance.